1. Field of the Invention
The invention relates to a pressure-sensitive adhesive composition, a pressure-sensitive adhesive layer, and a pressure-sensitive adhesive sheet. More specifically, the invention relates to an acrylic pressure-sensitive adhesive composition having a long pot life, a pressure-sensitive adhesive layer produced by crosslinking the composition, and a pressure-sensitive adhesive sheet including a support and such a pressure-sensitive adhesive layer formed on the support.
2. Description of the Related Art
Pressure-sensitive adhesive sheets are now increasingly being used in a variety of processes, such as fixing (bonding), transporting, protecting, and decorating of articles. Typical examples of such pressure-sensitive adhesive sheets have a pressure-sensitive adhesive layer that is formed using a pressure-sensitive adhesive composition containing an acrylic polymer as a base polymer (an acrylic pressure-sensitive adhesive composition). Such an acrylic polymer is usually a copolymer of an alkyl(meth)acrylate (a main component) and another monomer having a suitable functional group. Such an acrylic polymer having a functional group is mixed with a crosslinking agent reactive with the functional group, so that a pressure-sensitive adhesive composition is obtained. A pressure-sensitive adhesive layer is formed by crosslinking the acrylic polymer.
Removable acrylic pressure-sensitive adhesive sheets are used for surface protection, masking tapes, electrical and electronic devices, optical films, automobiles, etc. If the cohesive strength of the pressure-sensitive adhesive layer is insufficient in such removable acrylic pressure-sensitive adhesive sheets, a problem will occur in which the pressure-sensitive adhesive forms a residue on the surface of the adherend during the removal of the sheets. To solve this problem, the pressure-sensitive adhesive layer is required to have high cohesive strength and thus to be highly crosslinked. On the other hand, crosslinking, which is a chemical reaction, proceeds with time and needs a time period to be stabilized. The adhesive strength also changes as the crosslinking proceeds. This means that the crosslinking reaction should be completed at an early stage. Thus, for example, when an acrylic copolymer having a hydroxyl group is combined with an isocyanate crosslinking agent, a metal catalyst such as a tin compound is used (see JP-A-2005-314513). Unfortunately, in view of environmental issues these days, there have been concerns or regulations on the use of specific metals.
A pressure-sensitive adhesive composition for use in forming a pressure-sensitive adhesive layer is sometimes stored for a certain period of time depending on the production planning, and at the storage stage, the pressure-sensitive adhesive composition may be crosslinked. If crosslinking proceeds during the storage of the pressure-sensitive adhesive composition, the composition can increase in viscosity or form an insoluble product, which will cause roughening of the surface or variations in the thickness of a pressure-sensitive adhesive layer being formed subsequently. This may also affect the adhesive properties. Thus, there has been a demand for a pressure-sensitive adhesive composition that does not undergo crosslinking during storage, has a sufficiently-long useful life, called “pot life,” and can be rapidly crosslinked in the process forming a pressure-sensitive adhesive layer.